vacuum science and technology for particle accelerators 2019 … · uspas vacuum (june 17-21, 2019)...

Vacuum Science and Technology

for Particle Accelerators

Yulin LiCornell University, Ithaca, NY

Xianghong LiuSLAC National Accelerator Laboratory

Albuquerque, New MexicoJune 17-28, 2019

Table of Contents

2

Vacuum Fundamentals

Sources of Gases

Vacuum Instrumentation

Vacuum Pumps

Vacuum Components/Hardware

Vacuum Systems Engineering

Accelerator Vacuum Considerations, etc.

Beam-vacuum interactions

SESSION 5.2: VACUUM JOINTS

• Permanent Joints Welding

Brazing and Soldering

Bonding

• Removable Joints All-metal flanges

Elastomer flanges

Dynamic joints

3

USPAS Vacuum (June 17-21, 2019) 4

Methods of Making Vacuum Joints

Fluxed C

onsu

mable

Elect

rode

Vacuum Joints

Permanent

Non-Metal

Cera

mic-ce

ramic

Cera

mic-meta

l

Glass

-glass

Glass

-meta

l

Cera

mic-glass

Metal

Bra

zing

Diffu

sion

Bon

ding

Soldering

Welding

TIG

Resist

anc

e

Plasm

a A

rc

MIG

Elect

ron

Beam

Explos

ive

Lase

rGre

ase

Seals

Meta

l Seals

Elast

omer

Seals

Removable


Welding

Welding is the process where two materials are joined by fusion

Welding is the most common method for joining metals in vacuum systems.

Inert gas welding is the most common type of welding (TIG, MIG).

Joint design is critical from vacuum, metallurgical and distortion standpoints.

Cleanliness is essential.

Other welding processes to consider are electron beam and laser welding.


TIG – Tungsten Inert Gas Welding

TIG welding is one of most difficult welding, when operating manually. Inert gas mixture (Ar and He) form a shield to protect the hot-zone from oxidization. The

composition of the gas mixture vary with metal and mass. Arc power may be direct current or alternate current Filler rod is often used, but not needed for some fusion welds.


MIG – Metal Inert Gas Welding

Unlike TIG welding, the filler metal is a part of welding circuit.

MIG welding is much easier, cheaper and faster

But MIG welds are not as reliable and not nearly as clean as TIG.

So MIG welding is generally not a choice for vacuum welds


Welding Stainless Steel

TIG welding of stainless steels is among easiest, and heat-affected zone (HAZ) is very small.

In most cases, filler is not needed. However, reinforce stitch welds with filler are added for strengthening the welded joint.


Welding Copper

• The high thermal conductivity of copper makes welding difficult. Heating causes the copper to re-crystalize forming large grain size and annealing. Distortion is also a big problem.

• Copper weldment can be fused via TIG with or without fillers.

• Braze copper to copper, or copper to stainless steel is also done with TIG technique, using CuSil (72%Ag-28%Cu) alloy fillers. (At Cornell, we call this BT-weld, or Braze-TIG.)


Welding Copper

• Copper requires:

1. Very high welding speeds

2. Excellent material purity (OFE copper) and cleanliness.

3. Good joint design

4. Welding coppers must be done in an inert glove-box, or at least purging in-vacuum surfaces.

• Electron beam welding is an excellent process for welding copper.

• Vacuum furnace braze is also a good option for coppers.


Welding Aluminum Low melting point, relatively high thermal conductivity, and high rate of

thermal expansion make welding aluminum more problematic than stainless steel.

Aluminum requires:

High welding speeds (higher current densities)

Good material purity and cleanliness

Good joint design

Aluminum welds have a tendency to crack from excessive shrinkage stresses due to their high rate of thermal contraction. Filler rods (usually 4043 or similar alloys) always needed.

Due to relatively large weld-beads, out-side welding seams are very common for aluminum welds

To keep the aluminum weldment clean from oxidization, AC power is usually used during TIG, with characteristic popping noises.


Welding Aluminum – ExamplesV-groove for filler

containment


Electron Beam Welding (EBW)

• EBW provides extremely high energy density in its focused beam producing deep, narrow welds.

• This rapid welding process minimizes distortion and the heat affected zone.

• Very good control and reproducibility in weld penetration.

• A disadvantage of EBW is that the process takes place under vacuum (P = 10-5 Torr):

— Extensive fixturing required

— High initial cost

— Weld preps are extremely critical, as no filler used.

— Complexity

— Welds are not cleanable


Copper chambers EBW

RF CavityLoad into EBW Chamber E-beam welding

Welding 1-mm thick Copper Coveron CesrTA EC Detector Chamber


SLAC Electron Beam Welder


SolderingSoldering is the process where materials are joined together

by the flow of a “filler metal” through capillary action.

Soldering is differentiated from brazing primarily by the melting temperature of the filler metals. Solder alloys melt below 450C.

Because of lower working temperature, usually corrosive flux is needed to ensure proper ‘wetting’ of the surfaces.

All soft solders are unacceptable for UHV systems because: They contain Pb, Sn, Bi, Zn (vapor pressures are too high) System bake-out temperatures typically exceed alloy melting points. Residuals of corrosive flux left on the joints, a long-term reliability issue.

Most silver solders are unacceptable.


BrazingBrazing is the process where two dissimilar materials are joined together by the flow of a “filler metal” through capillary action.

There are several different brazing processes:1. Torch

2. Furnace

3. Induction

4. Dip

5. Resistance

Brazing can be used to join many dissimilar metals. The notable exceptions are aluminum and magnesium.

Cleanliness is important in brazing. Cleanliness is maintained by use of a flux or by controlling the atmosphere (vacuum or H2).

For vacuum furnace brazing, flux is NOT used.


Capillary ‘Wetting’ Rely on Uniform and Clean Narrow Gap

Contaminated

Surface

Capillary Action

Stops at larger gap


Vertical Capillary Test Specimen


Brazing (Cont.)

• Filler metals (brazing alloys) come in the form of wire, foils, or paste.

• Brazing alloys are selected to have melting points below that of the base metal. The brazing alloys must be able to ‘wet’ the base metal(s).

• Generally, brazing alloys can be categorized into eutectic or non-eutectic. For eutectic alloy, the transitions between solidus and liquids occur at a very narrow band (within a degree). Eutectic alloys are usually preferable.

• Multiple braze steps are possible by choosing alloys of differing melting points and proceeding sequentially from highest to lowest temperature.

• Braze joints require tight tolerances for a good fit (0.002” to 0.004”) to ensure capillary flow.


Some Braze Alloys for UHV Components

AlloyBrazing

Temperature Composition

GeoroTM 361C 88% Au, 12% Ge

CuSilTM 780C 72% Ag, 28% Cu

BAu -2 890oC 80% Au, 20% Cu

Au-Cu-Ni 925oC 81.5% Au, 16.5% Cu, 2% Ni

BAu -4 950oC 82% Au, 18% Ni

50/50 Au-Cu

970oC 50% Au, 50% Cu

35/65 Au-Cu

1010oC 35% Au, 65% Cu


Braze Alloy Phase Diagram Examples


Vacuum Braze Joints – Examples

CESR Beryllium Injection Windows

Post-brazing machining

720C980C


Permanent Joint Design Considerations

Compatibility between materials to be jointed (welding, brazing etc.)

Weld (joint) preps – Consult with your welder(s)

Seam accessibility – Consult with your welder(s)

Leak checking procedures and fixtures – Consult with your vacuum technicians

Weld fixability – Any joint may be leaky

All parts MUST be UHV cleaned before any welding

USPAS Vacuum (June 17-21, 2019)

• The explosion bonding (EXB, or EXW) process is a solid-state joining process used to join a wide variety of materials that cannot be joined by traditional fusion welding processes.

• The basic components consist of a base metal, a cladmetal above the base metal (with a small uniform gap)and the explosive charge on top of the clad metal. However, ductile metal interlayer(s) is needed to facilitate reliable bonding between the base and theclad metals, such as aluminum alloys to stainless steels.

• A controlled explosive detonation accelerates theclad metal into the base metal at a sufficient speedand causes the metals to fuse together.

• The force of the explosion sets up an angular collision,producing a plasma jet that removes impurity, leavingclean metal surfaces for joining.

• The pressure at the collision point (700~4000 MPa) causes the metals to behavior like viscous fluids, as evident by the wave-pattern at the bonding zone.

25

Explosion Bonding (Welding)

../../../PO Related/Al-SST/PAE Explosion Bonding and Forming/PAE EXPLOSIVE WELDING.pdf


Explosion Bonding – Cont.

EXB components made to drawings:

• Atlas Technologies

(https://www.atlasuhv.com/)

• Thermionic Vacuum Products

(https://thermionics.com/products/)

• High Energy Metals, Inc.

EXB plates available from:

• PA&E (http://pacaero.com/products/explosive-bonded-metals/)

• High Energy Metals, Inc.

(http://highenergymetals.com/)Explosive Forming

EXP BONDING.avi

https://www.atlasuhv.com/

https://thermionics.com/products/

http://pacaero.com/products/explosive-bonded-metals/

http://highenergymetals.com/

EXP FORMING.avi


Atlas Technologies Bonding Matrix Copy Right Atlas Technologies January 1998

Alum

inum

AL. A

lloy

Chro

miu

m

Copp

er

CU A

lloy

Glid

Cop

Gol

d

Hafn

ium

Indi

um

Iron

Lead

Mag

nesi

um

Mol

ydbe

num

Mol

y. A

lloy

Nick

el, (

Inva

r)

Niob

ium

Plat

inum

Rhen

ium

Silve

r

Stee

l, &

Allo

ys

Stee

l, M

ild

Stai

nles

s St

eel

Tant

alum

Tin

Tita

nium

Tung

sten

Vana

dium

Zinc

Zirc

oniu

m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Aluminum 1

AL. Alloy 2

Chromium 3

Copper 4

CU Alloy 5

Gold 6

GlidCop 7

Hafnium 8

Indium 9

Iron 10

Lead 11

Magnesium 12

Molydbenum 13

Moly. Alloy 14

Nickel, (Invar) 15

Niobium 16

Platinum 17

Rhenium 18

Silver 19

Steel, & Alloys 20

Steel, Mild 21

Stainless Steel 22

Tantalum 23

Tin 24

Titanium 25

Tungsten 26

Vanadium 27

Zinc 28

Zirconium 29

Bonding Capability

Flange Metal Standards Beam Stop, Absorber Materials Super-conducting Flange Materials

A Claim – All Metal Combinations can be bonded


Explosion Bonding – Examples

Ref: PA&E Bonded

Metals Division


SS/AL Bond Interface

Stainless

Aluminum

Stainless

Copper

Titanium

AL/SS Bond Interface

• Diffusion Inhibiting Layers: Copper and Titanium Interlayer enables Bonding AL/SS

• Vacuum: <1x10-10cc He/Sec

• Thermal: Peak 500oC at weld up 0-250oC Operational

• Mechanical: Tensile 38,000 psi; Shear 30,000 psi

Ref: Atlas Technologies

January 19-23 2015


Applications for bi-metallic joints

Alum/SST transition enable welding

instrument feedthroughs to aluminum

beampipe

Cu/SST transition used on Cesr-C

damping wiggler beampipes


SS/AL Bonding – Tests and QCs SST/AL transition is most commonly used on accelerator UHV systems. However, the SST/AL bonding is

among most difficult combination. The bonding usually involve one or more interlayer materials, such as Cu, Ti, Nb, just to name a few.

Some tests are specified in testing the bonding quality during procurement of SST/AL materials and components, including: Tensile strength test of the bond – e.g. Ram-tensile test Bonding void inspection – such as ultrasonic test Leak test – Sample specimen leak checks or component leak check

Some recent difficulties experienced by Cornell and NSLS II projects wereoccurred when leaks (often very large) were only detected after componentscompletion of welding of the SST/AL to the final assembly (!!), the worst scenario. The failed material usually involve the entire bonded

plate. The possible causes of failure including (1) theinterlayer(s) material(s) too thin; (2) the bondedplate is too large

At Cornell, we have included a welding test as a part of procurement specification, as shown to the left.


Diffusion bonding is a joining technique where pre-machined components are held together under modest loads at elevated temperatures.— The loads are usually well below those producing deformation.— Bonding temperatures typically range from 50-80% of melting temperatures of the metals.— Processing times vary from 1 minute to over an hour.— Most diffusion bonding operations are conducted in vacuum or in an inert gas atmosphere

Diffusion bonding requires very clean components with excellent surface finishes.

Diffusion bonding can also bond dissimilar materials

Diffusion Bonding


Hot Isostatic Press – Metal Transitions Hot Isostatic Press, HIP, is a solid-phase process

that involves the application of a high-temperature,

high pressure gas (typically Argon) to components.

Unlike diffusion-bonding, HIP allows the welding of

more complex geometries. HIP relaxes surface

finish requirements on the mating facets.

Typical HIP process takes 6 to 16 h, but many

components can be fitted into one furnace.

Comparing Alum/SST, price is similar to ExB

material, ~$30/sq.in.

TPS and Super-KEKB vacuum system used

extensively of HIP bi-metal material, without failure,

but they had problem with ExB bi-metal flanges.

Metal Technology Co. is one of company in Japan

perform HIP services.

W. A. Bryant, Welding J., Dec 1975, p433-s

http://www.kinzoku.co.jp/english/index.html?lid=1


Hot Isostatic Press – Simple Examples

Cu/SST (Metal Technology)

Alum/SST (Injection Flanges)

(Taiwan Photon Source)


One HIP Example – SuperKEKB’s Collimator

A

B

C

A: Inner cooling

loop welded

B&C: Two Cu blades

and W tip clamped

The entire collimator blade

was formed by HIP


Friction Bonding/Welding

Friction welding (FW) is a class of solid-state welding processes that generates heat through mechanical friction between a moving work piece and a stationary component, with the addition of a lateral force called "upset" to plastically displace and fuse the materials.

FW is useful to bond dissimilar materials, such as aluminum to stainless steel, which otherwise difficult to joint.

100s used on CESR SLDJTs


Demountable Metal Sealed Joints

There are a variety of metal seals available for vacuum systems, including:

ConFlat flanges: for flanges OD <26”

Helicoflex seals: for customer designed flanges

Metal wire seal flanges: for large flanges

VATSEAL flanges: good RF properties

Metal seals are used for UHV systems, where permeation, as well as radiation damages, are not acceptable.


Conflat® Flanges

Conflat style metal seal flanges are the most widely used for UHV vacuum systems.

During sealing, knife-edges of a pairing stainless steel flanges plastically deform a copper gasket to form a reliable seal.

The close match of C.T.E. of stainless steel and copper ensure proper sealing force through temperature cycles, up to 450C.

Gaskets are usually made of ½-hard OFHC. Silver-plated Cu gaskets are used for system baked at temperature higher than 250C.

Standard sizes range from 1-1/3” to 16” OD, with fixed and rotatable styles.


Conflat® Flanges – Non-Stainless Steel

To avoid transition to stainless steel, CF flanges made of aluminum alloys may be

directly welded to aluminum chambers. At least two styles are available

commercially.

1. Base material in A2219 alloy with CrN hardening coating

on knife-edge. Need to use aluminum gaskets for seal,

and knife-edge cannot be repaired.

2. 6xxx-T6 alloy without hard coating (AluVac), using Al

or annealed Cu gaskets.

CF flanges could also be made of Cu alloys, such ZrCrCu, for better thermal design

for applications dealing with high power density (such as photon-absorbers.)

Knife Edge Stability of CF Components made of Lightweight

Aluminum

https://www.vacom.de/en/downloads/white-papers

https://www.vacom.de/en/downloads/white-papers?download=3003:knife-edge-stability-of-cf-components-made-of-lightweight-aluminum


Aluminum Conflat® Flanges @ CBETA

Over 100 2-3/4” CF flanges, made of 6013-T8 alloy, were used on the

vacuum system of CBETA (Cornell Brookhaven ERL Test Accelerator)

• Use commercially

available 1100-H14

gaskets.

• Tested with repeated

sealing cycles and

bakeouts (150C)


Conflat® Flanges Cont.


Helicoflex® Flanges Metal O-ring using an internal spring to maintain the seal force, while outer layer of the

gasket deforms to make seal.

Vacuum rated to 1 x 10-13 Torr; Temperature rated to 450C.

Typical size range: 10” - 20” od

Ref: http://www.techneticsgroup.com


Commercial Wire Seal Flanges Vacuum rated to 1 x 10-13 Torr, Temperature rated to 450C with copper wire gasket.

Typical size range: 20” – 27” od with >30” possible

Warning – male and female flanges


Large Wire Seal Flanges at CHESS

As with many X-ray light sources, it is always a challenge to make reliable UHV-compatible seal on a very large lid (often in vertical position) for X-ray optics, as elastomer seal is not acceptable.

A reliable, inexpensive and very scalable aluminum wire seal scheme was developed at CHESS, and also adapted by NSLS II.


1 1

2

2 3

Large Wire Seal Flanges at CHESS Cont.

How the seal is made:1. Four aluminum wires are stretched across sealing

surfaces, and tensioned with springs.2. The large lid is placed (with exaggerated bending)

over the wires. Tightening from center towards to the corners, to ensure that the wires kept stretched .

3. Under cuts at corners may improved sealing reliability.

Features of the seals:

1. No ‘upper-size’ limit.2. Reproducible and reliable seals3. Relatively relax seal surface finish, but need to be

flat.4. Only work on flanges with straight sealing edges


VATSEAL® Flanges

A style of metal seal for vacuum, cryogenics and high temperature applications.

silver-plated or gold-plated copper gaskets

VATSEAL metal seals make a leak-tight seal and at the same time a reliable, low resistance RF contact


Elastomer Flanges

Elastomer (O-ring) sealed flanges found their uses in accelerators mostly in high-vacuum, or insolation vacuum for cryo-genic systems (such as superconducting magnet.

ASA Flanges

Typical size range: 1” to 12” dia.

ISO FlangesFlanges come in a variety of fastening styles:

- Kwik-flange- Rotatable & Non-rotatable- Double claw clamp- Banded clamps


Elastomer Seals for UHV Systems

In general, elastomer seals are not suitable for accelerator UHV systems for the following reasons: Permeation through O-ring seals can be significant in humid environment

(‘seasonal-effect’ was very visible at CESR in early days, with our old home-made GVs using O-ring bonnet seals.

Radiation hardening of elastomer seals will result in leaks.

However, elastomer seals have been used with successes, in following CESR examples: Use O-ring as gate through seals for CESR’s RF-shielded gate valves with oval

beam apertures. ~30 GVs in use in CESR for more than 20 years, no O-ring leak due to radiation hardening. This resulted in a very significant saving ascompared to all-metal special RF-shielded GVs.

Differentially pumped O-ring seals for (decommissioned) CLEO beryllium beampiperemote flange joints, aka the ‘magic’ flanges.


Elastomer Seals – The ‘Magic’ Flange

The Challenge: Making an UHV joint 3-m in-the-hole


Elastomer Seals – The ‘Magic’ Flange

Differentially pumped double O-ring sealed ‘magic’ flange

(a) Be-Cu spring for RF contact; (b) Primary Viton O-ring;

(c) Differential pumping; (d) Outer Viton O-ring;

(e) Electrically conductive elastomer (EcE) O-ring for RF shield

vacuum science and technology for particle accelerators 2019 … · uspas vacuum (june 17-21, 2019)...

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